Gas discharge lasers such as excimer lasers are well known light sources useful for integrated circuit lithography. These lasers typically include two elongated discharge electrodes (for example, about 30 cm in length) that are separated by about 5-20 mm to establish a discharge region between the electrodes. A high voltage pulse power source provides high voltage electrical pulses to produce discharges between the electrodes to create a gain region in a laser gas. A laser gas circulation system is generally employed to produce sufficient laser gas flow between the electrodes to remove from the discharge region substantially all of the heated, ionized gas volume and erosion debris particles produced by each discharge prior to the next succeeding discharge. For this purpose, it is typically desirable to establish a gas flow through the discharge region that is relatively uniform along the length of the electrodes. For this reason, cross-flow fans (also referred to in the art as tangential fans) have been used. For example, co-owned U.S. Pat. Nos. 6,023,486, 6,034,984, 6,061,376, 6,195,378, 6,144,686 and 6,765,946 disclose several cross-flow fan designs, each of which is hereby incorporated by reference herein.
Structurally, cross-flow fans include an elongated, somewhat cylindrical, impeller (also sometimes referred to as a squirrel cage rotor) which is rotated about a longitudinal axis by one or more motors. For example, the impeller may include a plurality of annularly shaped hubs that are spaced apart along the rotation axis and oriented orthogonal to (and substantially centered on) the rotation axis. For the impeller, each hub-pair may constitute an impeller segment and a number of blades may be provided connecting the hubs together at or near the periphery of each segment. In this manner, the blades surround a somewhat cylindrical internal impeller volume. In use, the impeller is typically disposed within and rotates relative to a flow guiding structure which establishes an intake side and a discharge side of the impeller. For some arrangements, this flow guiding structure may include one or more so-called flow cutoff members. When the fan impeller is rotated, laser gas is drawn through the blades into the internal cylindrical volume over the entire length of the fan impeller. Inside the impeller, the laser gas flow is diverted and accelerated by a vortex that is created by the rotation of the impeller. The laser gas then exits over the entire length of the impeller on the discharge side.
As the impeller blades pass the director(s), e.g. flow cutoff member, they may adversely affect laser performance in two ways. First, a mechanical vibration may be produced in the cut-off member structure that may be transmitted to the optical components defining the laser cavity. Second, the flow produced by the impeller may not be smooth, but instead, may consist of many small pressure pulses. Some of these pressure pulses may reach the gain volume where they may perturb the gain media's index of refraction. This perturbation, in turn, may result in an undesirable deterioration of one or more laser performance parameters such as spectral bandwidth, divergence, pulse-to-pulse energy stability, etc. In general, laser performance deterioration is more pronounced at discharge repetition rates which corresponding to the impeller's blade pass frequency and its sub-harmonics (each of which is a function of the impeller rotation speed and the number of blades distributed around the impeller's periphery). As used herein, the term “blade pass frequency” and its derivatives means the reciprocal of the time duration between successive passes of a blade by a stationary point during an impeller rotation. Applicant's have found that an impeller with an even number of blades may generate more undesirable sub-harmonics than an impeller with an odd number of blades, and moreover, impellers having a prime number of blades may generate fewer sub-harmonics than impellers having a non-prime number of blades.
With the above considerations in mind, Applicants disclose a cross-flow fan impeller and cross-flow fan system for a gas discharge laser.